In the known so-called steer-by-wire (SBW) systems that eliminate the mechanical linkage between the steering wheel and the steering mechanism of the front wheels such as that described in Japanese Kokai Patent Application No. 2002-225733, there is a backup clutch for mechanically connecting the steering wheel and the steering mechanism. In the event of a malfunction in some part of the SBW system, the clutch is quickly engaged to cancel SBW control, and the system is switched to an assistance control steering mode that can reduce the steering effort of the driver.